Publications

2012

• Entropy Generation across Earth's Collisionless Bow Shock
  
  • Parks G. K.
  • Lee E.
  • Mccarthy M.
  • Goldstein M. L.
  • Fu S. Y.
  • Cao J.B.
  • Canu Patrick
  • Lin N.
  • Wilber M.
  • Dandouras I.
  • Rème H.
  • Fazakerley A.
  Physical Review Letters, American Physical Society, 2012, 108, pp.061102. Earth's bow shock is a collisionless shock wave but entropy has never been directly measured across it. The plasma experiments on Cluster and Double Star measure 3D plasma distributions upstream and downstream of the bow shock allowing calculation of Boltzmann's entropy function H and his famous H theorem, dH/dt<=0. The collisionless Boltzmann (Vlasov) equation predicts that the total entropy does not change if the distribution function across the shock becomes nonthermal, but it allows changes in the entropy density. Here, we present the first direct measurements of entropy density changes across Earth's bow shock and show that the results generally support the model of the Vlasov analysis. These observations are a starting point for a more sophisticated analysis that includes 3D computer modeling of collisionless shocks with input from observed particles, waves, and turbulences. (10.1103/PhysRevLett.108.061102)
  DOI : 10.1103/PhysRevLett.108.061102
• Afterglow kinetics in oxygen pulsed discharges
  
  • Guerra V.
  • Marinov Daniil
  • Guaitella Olivier
  • Rousseau Antoine
  , 2012.
• Radiative properties of mixed nested cylindrical wire arrays on Zebra at UNR
  
  • Weller Michael E.
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Esaulov A. A.
  • Coverdale C. A.
  • Chuvatin Alexandre S.
  • Ouart N. D.
  • Williamson Kenneth M.
  • Shrestha Ishor
  • Osborne Glenn C.
  • Shlyaptseva V. V.
  • Keim S. F.
  • Stafford A.
  High Energy Density Physics, Elsevier, 2012, 8 (2), pp.184-189. The dynamics of mixed nested cylindrical wire arrays were studied at the UNR Zebra generator with our existing theoretical and experimental tools to better understand the contributions of each array to the emitted radiation. In particular, experimental results of mixed brass (70% Cu, 30% Zn) and Al (5056, 5% Mg) nested cylindrical wire arrays are analyzed and compared. The loads used brass in the inner array and Al in the outer array, or alternately, Al in the inner array and brass in the outer array, with a mass ratio of 1:1 (outer to inner). Consequently, radiative properties of K-shell Al and Mg ions and L-shell Cu and Zn ions are compared as functions of the placements of the brass and Al wires on the inner and outer arrays. Results show that the placement of brass and Al, whether on the inner or outer array, dramatically affects the intensity of the X-ray emission. Specifically, the ratio of Cu L-shell to Al K-shell emissions changed from 4 when Al is in the outer array to 40 when brass is in the outer array, and the total radiated yield was highest when the brass was on the outer array (18 kJ, versus 15 kJ when brass is on the inner array). Each load was fielded twice to vary the timing of the time-gated imaging and spectral diagnostics. This provides a more complete understanding of the evolution of the plasma parameters over the X-ray pulse and highlights the importance of the time-gated diagnostics. (10.1016/j.hedp.2012.03.004)
  DOI : 10.1016/j.hedp.2012.03.004
• Global model of a gridded-ion thruster powered by a radiofrequency inductive coil
  
  • Chabert Pascal
  • Arancibia Monreal J.
  • Bredin Jérôme
  • Popelier Lara
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.195201. A global (volume-averaged) model of a gridded-ion thruster is proposed. The neutral propellant (xenon gas) is injected into the thruster chamber at a fixed rate and a plasma is generated by circulating a radiofrequency current in an inductive coil. The ions generated in this plasma are accelerated out of the thruster by a pair of DC biased grids. The neutralization downstream is not treated. Xenon atoms also flow out of the thruster across the grids. The model, based on particle and energy balance equations, solves for four global variables in the thruster chamber: the plasma density, the electron temperature, the neutral gas (atom) density, and the neutral gas temperature. The important quantities to evaluate the thruster efficiency and performances are calculated from these variables and from the voltage across the grids. It is found that the mass utilization efficiency rapidly decreases with the gas flow rate. However, the radiofrequency power transfer efficiency increases significantly with the injected gas flow rate. Therefore, there is a compromise to be found between these two quantities. (10.1063/1.4737114)
  DOI : 10.1063/1.4737114
• Seasonal TEC Variability in West Africa Equatorial Anomaly Region
  
  • Zoundi Christian
  • Ouattara Frédéric
  • Fleury Rolland
  • Amory-Mazaudier Christine
  • Lassudrie-Duchesne Patrick
  European Journal of Scientific Research, EuroJournals, 2012, 77 (3), pp.309-319. This paper presented the seasonal variability of TEC/ GPS data recorded at Ouagadougou a West Africa GPS station located near the magnetic equator. Seasonal data TEC time variations are compared to those of TEC derived from IGS GPS network maps. The present study showed that TEC map model predicts well data TEC during equinoctial months and fairly well during solstice months. The best prediction is obtained during spring and the worst during winter. The analysis of seasonal TEC profiles highlighted that model accuracy shows seasonal variations with respect to the complexity of TEC time variations. This work pointed out that model accuracy depends not only on the integration of station data in IGS GPS network data but also on the presence of daytime multi-peaks and/or the presence and the amplitude of night time peak. Solstice and summer data TEC analysis showed that those present F2 layer annual anomaly.
• Transition from Weak to Strong Cascade in MHD Turbulence
  
  • Verdini Andrea
  • Grappin Roland
  Physical Review Letters, American Physical Society, 2012, 109, pp.025004. The transition from weak to strong turbulence when passing from large to small scales in magneto-hydrodynamic (MHD) turbulence with guide field is a cornerstone of anisotropic turbulence theory. We present the first check of this transition, using the Shell-RMHD, which combines a shell model of perpendicular nonlinear coupling and linear propagation along the guide field. This model allows us to reach Reynolds numbers around 106. We obtain surprisingly good agreement with the theoretical predictions, with a reduced perpendicular energy spectrum scaling as k^-2 at large scales and as k_⊥^-5/3 at small scales, where critical balance between nonlinear and propagation time is reached. However, even in the strong regime, a high level of excitation is found in the weak coupling region of Fourier space, which is due to the rich frequency spectrum of large eddies. A corollary is that the reduced parallel spectral slope is not a definite test of the spectral anisotropy, contrary to standard belief. (10.1103/PhysRevLett.109.025004)
  DOI : 10.1103/PhysRevLett.109.025004
• Electron and wave characteristics observed by the THEMIS satellites near the magnetic equator during a pulsating aurora
  
  • Nakajima A.
  • Shiokawa K.
  • Sakaguchi K.
  • Miyoshi Y.
  • Lee S.
  • Angelopoulos V.
  • Le Contel Olivier
  • Mcfadden J. P.
  • Bonnell J. W.
  • Fornacon K.-H.
  • Donovan E.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117, pp.3219. Based on conjugate ground and THEMIS satellite observations, we show electron spectra and wave characteristics near the magnetic equatorial plane during a pulsating aurora event on the high latitude side of the auroral oval. The pulsating aurora was observed by a 30-Hz sampled all-sky imager (ASI) at Gillam (56.4°N, 265.4°E), Canada, at 0840-0910 UT on 8 January 2008. The auroral intensity pulsation at the possible THEMIS D (THD) footprints had frequency peaks at 0.1-0.2 Hz. The footprint of THD was in the poleward part of the proton aurora observed by a meridian-scanning photometer. After auroral pulsation began at 0842 UT, both THD and THEMIS E which was near THD in the mid-tail at 11.6-11.8 R<SUB>E</SUB>, observed enhanced field-aligned electron fluxes at energies of 1-10 keV. However, the amplitudes of whistler mode waves and electrostatic cyclotron harmonics (ECH) waves observed by THD with the highest sampling rate of 8 kHz were not significant, showing a marked contrast to the recent report of clear correlation between whistler mode waves and auroral pulsations observed at 5-9 R<SUB>E</SUB>. We suggest that the observed field-aligned electrons, which are probably caused by Fermi-type acceleration associated with earthward plasma flow in the mid-tail plasma sheet, are modulated by some wave processes to cause pulsating auroras. (10.1029/2011JA017066)
  DOI : 10.1029/2011JA017066
• Electric and magnetic contributions to spatial diffusion in collisionless plasmas
  
  • Smets Roch
  • Belmont Gérard
  • Aunai Nicolas
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.102309. We investigate the role played by the different self-consistent fluctuations for particle diffusion in a magnetized plasma. We focus especially on the contribution of the electric fluctuations and how it combines with the (already investigated) magnetic fluctuations and with the velocity fluctuations. For that issue, we compute with a hybrid code the value of the diffusion coefficient perpendicular to the mean magnetic field and its dependence on the particle velocity. This study is restricted to small to intermediate level of electromagnetic fluctuations and focuses on particle velocities on the order of few times the Alfvén speed. We briefly discuss the consequences for cosmic ray modulation and for the penetration of thermal solar wind particles in the Earth magnetosphere. (10.1063/1.4762845)
  DOI : 10.1063/1.4762845
• Diagnostic emission and absorption spectroscopy
  
  • Roepcke J.
  • Rousseau Antoine
  • Lavrov B.P.
  • Davies P.B.
  , 2012.
• Shell-Reduced MHD: weak and strong turbulence
  
  • Grappin Roland
  • Verdini Andrea
  , 2012, 14, pp.3856. MHD turbulence with guide field B0 is studied using the shell model for Reduced MHD, with the aim of characterizing the scaling, and anisotropy of RMHD at high Reynolds number. A basic prediction of anisotropic turbulence theory (critical balance or CB) is that, in presence of large enough B0, the 1D perpendicular energy spectrum should exhibit a k&#8869;-2 scaling characteristic of weak nonlinear coupling at large scales, followed by a break and a k&#8869;-5/3 scaling at smaller scales. Previous numerical simulations never obtained such double spectra, but instead displayed single power-laws with a monotonous increase of their index when B0 was increased. In the present work we observe for the first time the double scaling, possibly thanks to the high Reynolds number reachable in shell models. A corollary of the CB theory is that a k||-2 spectrum develops in the range with strong k&#8869;-5/3 scaling, characterizing spectral anisotropy. In our study, we find that, although the basic paradigm of the CB are verified, the parallel spectrum differs, showing an over-excitation of parallel modes, due to high-frequencies of the large perpendicular eddies. This proves on the one hand that the CB theory is well adapted to the RMHD model but on the other hand that the model is richer than previously thought.
• Dynamic Martian magnetosphere: Transient twist induced by a rotation of the IMF
  
  • Modolo Ronan
  • Chanteur Gérard
  • Dubinin Eduard
  Geophysical Research Letters, American Geophysical Union, 2012, 39 (1), pp.L01106. Simulation studies of the Martian environment are usually restricted to stationary situations under various steady conditions of the solar wind and solar radiation. Dynamic transients and their implications have so far attracted little attention although global simulation models can provide valuable insights to understand disagreements between simulations and in situ observations. We make use of a three dimensional multispecies hybrid simulation model to investigate the response of the Martian plasma environment to a sudden rotation of the IMF. The simulation model couples charged and neutral species via three ionisation mechanisms: the absorption of solar extreme ultraviolet radiation, the impact of solar wind electrons, and the charge exchange between ions and neutral atoms. When a rotational discontinuity conveyed by the solar wind reaches the Martian environment the bow shock adapts quickly to the new solar wind conditions in contrast to the induced magnetosphere, especially the magnetic lobes in the wake. Timescales necessary to recover a stationary state can be estimated from such simulations and have some implications for space observations especially in the use of magnetic field proxies and for organizing particle measurements made by a spacecraft like Mars Express without an onboard magnetometer. (10.1029/2011GL049895)
  DOI : 10.1029/2011GL049895
• On time resolved gas temperature measurements in a pulsed dc plasma using quantum cascade laser absorption spectroscopy
  
  • Hübner M.
  • Marinov Daniil
  • Guaitella Olivier
  • Rousseau Antoine
  • Roepcke J.
  Measurement Science and Technology, IOP Publishing, 2012, 23, pp.115602. With a time resolution of 33 µs, the gas temperature in a pulsed dc air plasma admixed with 0.8% NO has been measured by quantum cascade laser absorption spectroscopy (QCLAS). For this purpose, the temperature dependent intensity ratios of two absorption structures of NO at 1900 cm&#8722;1 (5.26 µm) have been used. The QCLAS system worked in the Intra Pulse Mode with a pulse repetition frequency of 30 kHz leading to a spectrum recorded each 33 µs. In a low pressure discharge, the influence of nonlinear absorption phenomena causing strong distorted absorption structures of NO has been taken into account by a calibration routine based on tabulated line strengths. Different mean plasma currents have been applied to the discharge leading to gas temperature values ranging from about 300 K up to about 500 K. (10.1088/0957-0233/23/11/115602)
  DOI : 10.1088/0957-0233/23/11/115602
• Plasma decay in the afterglow of a high-voltage nanosecond discharge in air
  
  • Aleksandrov N.L.
  • Anokhin E.M.
  • Kindysheva S.V.
  • Kirpichnikov A.A.
  • Kosarev I.N.
  • Nudnova M.M.
  • Starikovskaia Svetlana
  • Starikovskii A.Yu.
  Fizika Plazmy / Plasma Physics Reports, MAIK Nauka/Interperiodica, 2012, 38 (2), pp.179-186. The decay of air plasma produced by a high-voltage nanosecond discharge at room temperature and gas pressures in the range of 110 Torr was studied experimentally and theoretically. The time dependence of the electron density was measured with a microwave interferometer. The initial electron density was about 10¹2 cm3. The discharge homogeneity was monitored using optical methods. The dynamics of the charged particle densities in the discharge afterglow was simulated by numerically solving the balance equations for electron and ions and the equation for the electron temperature. It was shown that, under these experimental conditions, plasma electrons are mainly lost due to dissociative and three-body recombination with ions. Agreement between the measured and calculated electron densities was achieved only when the rate constant of the three-body electronion recombination was increased by one order of magnitude and the temperature dependence of this rate constant was modified. This indicates that the mechanism for three- body recombination of molecular ions differs from that of the well-studied mechanism of atomic ion recombination. (10.1134/S1063780X12010011)
  DOI : 10.1134/S1063780X12010011
• Comparison of a hybrid model to a global model of atmospheric pressure radio-frequency capacitive discharges
  
  • Lazzaroni Claudia
  • Lieberman M.A.
  • Lichtenberg A.J.
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.495204. A one-dimensional hybrid analyticalnumerical global model of atmospheric pressure radio-frequency (rf) driven capacitive discharges, previously developed, is compared with a basic global model. A helium feed gas with small admixtures of oxygen is studied. For the hybrid model, the electrical characteristics are calculated analytically as a current-driven homogeneous discharge. The electron power balance is solved analytically to determine a time-varying Maxwellian electron temperature, which oscillates on the rf timescale. Averaging over the rf period yields effective rate coefficients for gas phase activated processes. For the basic global model, the electron temperature is constant in time and the sheath physics is neglected. For both models, the particle balance relations for all species are integrated numerically to determine the equilibrium discharge parameters. Variations of discharge parameters with composition and rf power are determined and compared. The rate coefficients for electron-activated processes are strongly temperature dependent, leading to significantly larger neutral and charged particle densities for the hybrid model. For small devices, finite sheath widths limit the operating regimes to low O2 fractions. This is captured by the hybrid model but cannot be predicted from the basic global model. (10.1088/0022-3727/45/49/495204)
  DOI : 10.1088/0022-3727/45/49/495204
• Tailored voltage waveform deposition of Microcrystalline Silicon thin films from hydrogen-diluted silane and silicon tetrafluoride: optoelectronic properties of films
  
  • Johnson E.V.
  • Pouliquen S.
  • Delattre Pierre-Alexandre
  • Booth Jean-Paul
  Japanese Journal of Applied Physics, Japan Society of Applied Physics, 2012, 51, pp.08HF01. The use of tailored voltage waveforms (TVW's) to excite a plasma for the deposition of thin films of hydrogenated microcrystalline silicon (µc-Si:H) has been shown to be an effective technique to decouple mean ion bombardment energy (IBE) from injected power. In this work, we examine the changes in material properties controlled by this technique through Raman scattering and spectroscopic ellipsometry for films deposited from H2-diluted SiH4, and we examine the electrical properties of such films using temperature dependent conductivity. As the laboratory-scale deposition system used had neither a load lock nor an oxygen filter in the H2 line, accidental O-doping was observed for the µc-Si:H films. We investigated suppression of this doping by adding varying amounts of SiF4, and using an SiF4/Ar pre-etch step to clean the reactor. This technique is shown to be effective in decreasing the accidental doping of the films, and intrinsic µc-Si:H films are produced with an activation energy of up to 0.55 eV. As well, an important difference in the amorphous-to-microcrystalline transition is observed once SiF4 is included in the gas mixture. (10.1143/JJAP.51.08HF01)
  DOI : 10.1143/JJAP.51.08HF01
• A hemispherical retarding field energy analyzer to characterize spatially and angularly extended electron beams
  
  • Cipriani Fabrice
  • Leblanc Frédéric
  • Illiano Jean-Marie
  • Berthelier Jean-Jacques
  European Physical Journal: Applied Physics, EDP Sciences, 2012, 60 (2), pp.21002 (7 p.). We have designed and built a hemispherical retarding field energy analyzer in order to facilitate characterization of large area electron emitters (typically field emitter arrays with active areas up to 1 cm2) with large angular aperture. A complete numerical model of the analyzer has been built, including perturbations due to secondary particles, in order to determine the analyzer performances. The analyzer energy resolution is better than 100 meV for an energy range up to 120 eV. The analyzer has a global field of view of 112° and allows measurements of the energy distribution of the beam as a function of the emission angle, as well as measurements of the beam intensity profile along any section of the beam. We have successfully used the analyzer to characterize the electron beam emitted by 1 cm2 Mo microtips-based field emitter arrays. (10.1051/epjap/2012120011)
  DOI : 10.1051/epjap/2012120011
• Kinetic equilibrium for an asymmetric tangential layer, Physics of Plasmas
  
  • Belmont Gérard
  • Aunai Nicolas
  • Smets Roch
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.022108. Finding kinetic (Vlasov) equilibria for tangential current layers is a long standing problem, especially in the context of reconnection studies, when the magnetic field reverses. Its solution is of pivotal interest for both theoretical and technical reasons when such layers must be used for initializing kinetic simulations. The famous Harris equilibrium is known to be limited to symmetric layers surrounded by vacuum, with constant ion and electron flow velocities, and with current variation purely dependent on density variation. It is clearly not suited for the magnetopause-like layers, which separate two plasmas of different densities and temperatures, and for which the localization of the current density j=n&#948;v is due to the localization of the electron-to-ion velocity difference &#948;v and not of the density n. We present here a practical method for constructing a Vlasov stationary solution in the asymmetric case, extending the standard theoretical methods based on the particle motion invariants. We show that, in the case investigated of a coplanar reversal of the magnetic field without electrostatic field, the distribution function must necessarily be a multi-valued function of the invariants to get asymmetric profiles for the plasma parameters together with a symmetric current profile. We show also how the concept of accessibility makes these multi-valued functions possible, due to the particle excursion inside the layer being limited by the Larmor radius. In the presented method, the current profile across the layer is chosen as an input, while the ion density and temperature profiles in between the two asymptotic imposed values are a result of the calculation. It is shown that, assuming the distribution is continuous along the layer normal, these profiles have always a more complex profile than the profile of the current density and extends on a larger thickness. The different components of the pressure tensor are also outputs of the calculation and some conclusions concerning the symmetries of this tensor are pointed out. (10.1063/1.3685707)
  DOI : 10.1063/1.3685707
• Inductance and near fields of a loop antenna in a cold magnetoplasma in the whistler frequency band
  
  • Korobkov S. V.
  • Kostrov A. V.
  • Gushchin M. E.
  • Zaboronkova T. M.
  • Krafft C.
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.093301. The influence of a magnetoplasma on the inductance of a circular loop antenna oriented perpendicular to the ambient static magnetic field and operated in the whistler frequency band is studied. Based on a strict electrodynamic approach, the analytical treatment of the antenna reactance is performed for a uniform rf current distribution along the antenna wire. Calculations are made for plasma parameters and operating frequencies typical for active ionospheric experiments and laboratory rf (helicon) sources of dense magnetized plasmas. It is shown that the plasma influence on the inductance of the loop antenna remains relatively weak, even for antennas with dimensions close to half of the longitudinal whistler wavelength, when the rf field distribution in the antenna near zone is strongly different from that in vacuum. The theoretical predictions are confirmed by measurements performed on the large KROT plasma device. The results obtained are of crucial importance for the preparation of active ionospheric experiments and for the matching of loop antennas used in laboratory rf sources of dense magnetized plasmas. (10.1063/1.4745611)
  DOI : 10.1063/1.4745611
• The International Research Group in geophysics, Europa Africa : a laboratory without borders in the Earth Science and Environment
  
  • Amory-Mazaudier Christine
  Journal of Life Sciences, 2012, 6, pp.336-342. ...
• Determination of TEC by using pseudo range at Koudougou station in Burkina Faso
  
  • Ouattara Frédéric Martial
  • Zoundi C.
  • Amory-Mazaudier Christine
  • Fleury Rolland
  • Lassudrie Duchesne P.
  Journal des sciences, Université Cheikh Anta Diop, 2012, 11 (1), pp.12-19. ...
• Variation of F2 layer critical frequency with solar cycle at Dakar station
  
  • Thiam N. M.
  • Ouattara Frédéric Martial
  • Gnabahou Doua Allain
  • Amory-Mazaudier Christine
  • Fleury Rolland
  • Lassurdie-Duchesne P.
  Journal des sciences, Université Cheikh Anta Diop, 2012, 11 (2), pp.16-20. ...
• New Insight into Short-wavelength Solar Wind Fluctuations from Vlasov Theory
  
  • Sahraoui Fouad
  • Belmont Gérard
  • Goldstein M. L.
  The Astrophysical Journal, American Astronomical Society, 2012, 748, pp.100. The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is being investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with omega < omega<SUB>ci</SUB>, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., omega > omega<SUB>ci</SUB>) is more relevant. Here, we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically beta<SUB> i </SUB> >~ beta<SUB> e </SUB> ~ 1 and for high oblique angles of propagation 80° <= Theta<SUB> kB </SUB> < 90° as observed from the Cluster spacecraft data. The linear properties of the plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (Theta<SUB> kB </SUB> < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales krho<SUB> i </SUB> >~ 1 to frequencies either larger or smaller than omega<SUB>ci</SUB>, depending on the anisotropy k <SUB>par</SUB>/k <SUB></SUB>. This extension into small scales is more readily called whistler (omega > omega<SUB>ci</SUB>) or KAW (omega < omega<SUB>ci</SUB>), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW. (10.1088/0004-637X/748/2/100)
  DOI : 10.1088/0004-637X/748/2/100
• Thin current sheets in the presence of a guiding magnetic field in Earth's magnetosphere
  
  • Malova H. V.
  • Popov V. Y.
  • Mingalev O. V.
  • Mingalev I. V.
  • Melnik M. N.
  • Artemyev A. V.
  • Petrukovich A. A.
  • Delcourt Dominique C.
  • Shen C.
  • Zelenyi L. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117, pp.4212. A self-consistent theory of relatively thin anisotropic current sheets (TCS) in collisionless plasma is developed, taking into account the presence of a guiding field B<SUB>y</SUB> (all notations are used in the GSM coordinate system). TCS configurations with a finite value of guiding field B<SUB>y</SUB> are often observed in Earth's magnetotail and are typical for Earth's magnetopause. A characteristic signature of such configurations is the existence of a magnetic field component along the direction of TCS current. A general case is considered in this paper with global sheared magnetic field B<SUB>y</SUB> = const. Analytical and numerical (particle-in-cell) models for such plasma equilibria are analyzed and compared with each other as well as with Cluster observations. It is shown that, in contrast to the case with B<SUB>y</SUB> = 0, the character of ``particle-current sheet'' interaction is drastically changed in the case of a global magnetic shear. Specifically, serpentine-like parts of ion trajectories in the neutral plane become more tortuous, leading to a thicker current sheet. The reflection coefficient of particles coming from northern and southern sources also becomes asymmetric and depends upon the value of the B<SUB>y</SUB> component. As a result, the degree of asymmetry of magnetic field, plasma, and current density profiles appears characteristic of current sheets with a constant B<SUB>y</SUB>. In addition, in the presence of nonzero guiding field, the curvature current of electrons in the center of the current sheet decreases, yielding an effective thickening of the sheet. Implications of these results for current sheets in Earth's magnetosphere are discussed. (10.1029/2011JA017359)
  DOI : 10.1029/2011JA017359
• ON THE NATURE OF THE SOLAR WIND FROM CORONAL PSEUDOSTREAMERS
  
  • Wang Y-M
  • Grappin Roland
  • Robbrecht E.
  • Sheeley N R
  The Astrophysical Journal, American Astronomical Society, 2012, 749 (2), pp.182. Coronal pseudostreamers, which separate like-polarity coronal holes, do not have current sheet extensions, unlike the familiar helmet streamers that separate opposite-polarity holes. Both types of streamers taper into narrow plasma sheets that are maintained by continual interchange reconnection with the adjacent open magnetic field lines. White-light observations show that pseudostreamers do not emit plasma blobs; this important difference from helmet streamers is due to the convergence of like-polarity field lines above the X-point, which prevents the underlying loops from expanding outward and pinching off. The main component of the pseudostreamer wind has the form of steady outflow along the open field lines rooted just inside the boundaries of the adjacent coronal holes. These flux tubes are characterized by very rapid expansion below the X-point, followed by reconvergence at greater heights. Analysis of an idealized pseudostreamer configuration shows that, as the separation between the underlying holes increases, the X-point rises and the expansion factor f ss at the source surface increases. In situ observations of pseudostreamer crossings indicate wind speeds v ranging from ~350 to ~550 km s1, with O7 /O6 ratios that are enhanced compared with those in high-speed streams but substantially lower than in the slow solar wind. Hydrodynamic energy-balance models show that the empirical v-f ss relation overestimates the wind speeds from nonmonotonically expanding flux tubes, particularly when the X-point is located at low heights and f ss is small. We conclude that pseudostreamers produce a "hybrid" type of outflow that is intermediate between classical slow and fast solar wind. (10.1088/0004-637X/749/2/182)
  DOI : 10.1088/0004-637X/749/2/182
• Observations of turbulence within reconnection jet in the presence of guide field
  
  • Huang S. Y.
  • Zhou M.
  • Sahraoui Fouad
  • Vaivads A.
  • Deng X. H.
  • André M.
  • He J. S.
  • Fu H.S.
  • Li H. M.
  • Yuan Z. G.
  • Wang D. D.
  Geophysical Research Letters, American Geophysical Union, 2012, 39, pp.L11104. We present the first comprehensive observations of turbulence properties within high speed reconnection jet in the plasma sheet with moderate guide field. The power spectral density index is about &#8722;1.73 in the inertial range, and follows the value of &#8722;2.86 in the ion dissipation range. The turbulence is strongly anisotropic in the wave-vector space with the major power having its wave-vector highly oblique to the ambient magnetic field, suggesting that the turbulence is quasi-2D. The measured dispersion relations obtained using the k-filtering technique are compared with theory and are found to be consistent with the Alfvén-Whistler mode. In addition, both Probability Distribution Functions and flatness results show that the turbulence in the reconnection jet is intermittent (multifractal) at scales less than the proton gyroradius/inertial lengths. The estimated electric field provided by anomalous resistivity caused by turbulence is about 3 mV/m, which is close to the typical reconnection electric field in the magnetotail. (10.1029/2012GL052210)
  DOI : 10.1029/2012GL052210